In fluid handling systems, especially where extremely high pressures are encountered, for example, pressures in the order of 10,000 psi, the valve mechanisms that are typically employed in such service typically take the form of gate valves. In some cases it is desirable to provide a high pressure valve mechanism having not only a positive sealing capability but also a capability of relieving pressure in the event an opposing pressure surge should develop. Gate valves are typically utilized under circumstances where positive sealing capability is desired and are not used under circumstances where a back surge relieving capability is also desired. Gate valves typically are positionable only in open or closed positions and when closed establish positive sealing with respect to flow from either direction.
In order to accomplish a check valve function for the purpose of relieving surges of pressure or to provide for unidirectional flow through a conduit system controlled by a valve mechanism, check valve mechanisms are typically employed. In most cases, however, check valves only function to provide unidirectional flow control and are not capable of functioning in the conventional open and closed positions in order to provide for bidirectional flow control. Moreover, most check valve mechanisms are of low pressure characteristics and would not typically be employed under circumstances where fluid pressures are in the order of 10,000 psi or higher. It is desirable, therefore, to provide a valve mechanism having the capability of withstanding high pressure conditions and also providing the capability of functioning to provide bidirectional fluid control and to selectively provide a relieving function in the event reverse pressure surges should occur. It is also desirable to provide a valve mechanism having the capability of establishing positive sealing in the event a check valve relieving function is not desired.
Especially under circumstances where liquid is being handled by a flow line system that is controlled by means of gate valves, it is possible for sections of the flow line system to be isolated between two closed gate valves. In some cases, flow conduit systems that are so isolated can develop extremely high pressures in response to heating and cooling thereof by the ambient environment. For example, when an isolated liquid flow line becomes heated, heat expansion of the liquid material can cause extremely high pressure conditions to be developed. It is desirable, therefore, to provide means for relieving pressures that develop in this manner so as to provide the flow system with a pressure responsive safety mechanism to protect equipment, personnel and the environment.